Electrical capacitance tomography ECT is one specific field within the more general field of electrical tomography. ECT as such is a known technique allowing non-invasive monitoring of a target domain on the basis of determination of the permittivity within the target domain.
In general, ECT comprises providing a model of the target domain and the measurement arrangement, making capacitance-related measurements, and adjusting the mathematical model so as to reduce the differences between the simulated and the measured electrical quantity values until a sufficient consistency exist, after which the permittivity in the target domain is determined. Typically, this is implemented by generating an image of the permittivity distribution in the target domain. Permittivity, and in particular changes thereof provide information on the internal material properties and distributions within the target domain.
A typical example of utilization of ECT is imaging a multi-phase flow in an industrial process, wherein an image showing the areas or volumes of different phases within the material flow is generated. An example of this kind of method and different practical issues involved therein is discussed in U.S. Pat. No. 7,496,450 B2.
Recently, the inventors have found it being possible to use ECT also e.g. for monitoring scaling (fouling) of undesired deposit on, as well as possible wear of process equipment surfaces in various industrial processes.
Taking into account the basic principle of ECT lying on measuring capacitance or, more generally, some electrical quantity of interest proportional to capacitance, the measurement sensor design and the actual measurement setup plays an important role in reconstructing the permittivity within the target domain. Most commonly, an ECT sensor is implemented as a tubular body forming a part of the process pipe or vessel, the content of which is to be analyzed. The electrodes are arranged in an annular assembly along the tubular body wall. As is clear from the basic nature of a plate capacitor, the capacitance thereof being proportional to the size of the capacitor plates, the electrodes should be sufficiently large in order to ensure sufficiently strong signals. This is important, for example, to achieve a sufficient signal to noise ratio. On the other hand, the spatial resolution in reconstructing the permittivity distribution naturally deteriorates when the electrode size is increased. As a compromise, the circumference of the tubular sensor body is typically divided for 8 to 12 equally sized electrodes.
Given the compromised situation described above, it is clear that new solutions for improving the signals to be measured would be highly desired.